There are a number of existing satellites in orbit today carrying one or more transponders that were designed for access from a single high-power ground antenna at a relatively low data rate (for example 256 kilobits per second). These transponders have a relatively narrow bandwidth and were originally designed for data relay at either half of full duplex. The transponders employ the space ground link system (SGLS) communications format. In one type of SGLS System a subcarrier is modulated by an information signal using BPSK or QPSK modulation. Then the composite signal phase modulates the primary carrier. The frequency spectrum for SGLS technique is illustrated in FIG. 3. In one embodiment the downlink subcarrier is at 1.7 MHz on a 1839.795 MHz primary carrier, and the uplink subcarrier is at 1.25 MHz on a 1795.732 MHz primary carrier.
Candidate data traffic using these existing transponders includes: secure voice or data; image dissemination, intelligence collection, telemetry and command relay, mission data relay, and computer-to-computer data transfer. Potential users may also require a wide range of communication connectivity schemes such as: transmit and receive, receive only, interconnection to many ground users in a broadcast mode, or collection schemes.
These existing satellites require a large received carrier power to keep the satellite coherent phase demodulator locked. This characteristic prevents the usual trade-off of reducing the required transmitter effective radiated power by reducing the transmission data rate. It has been shown that an effective radiated power of approximately 80 to 90 dBm is required to keep the phase demodulator locked. Generating this kind of power requires the use of a relatively large terminal, even for very low data rates.
It would be advantageous to allow users access to these existing satellite transponders from small tactical ground terminals having a diameter of approximately four feet. Using a terminal of this size requires approximately 500 watts of transmitted RF power to keep the phase demodulator of the existing transponders in the locked mode. According to the prior art, such a high power requirement would render the use of these satellites from a tactical terminal impractical.